Okay, so I uninstalled node-red and re-ran the installer with switches --node18 and --nodered-version="3.1"
The service now starts without a problem:
Starting as a systemd service.
6 Jul 19:24:08 - [info]
Welcome to Node-RED
===================
6 Jul 19:24:08 - [info] Node-RED version: v3.1.11
6 Jul 19:24:08 - [info] Node.js version: v18.20.3
6 Jul 19:24:08 - [info] Linux 6.6.31+rpt-rpi-v6 arm LE
6 Jul 19:24:12 - [info] Loading palette nodes
6 Jul 19:24:25 - [info] Settings file : /home/pi/.node-red/settings.js
6 Jul 19:24:25 - [info] Context store : 'default' [module=memory]
6 Jul 19:24:25 - [info] User directory : /home/pi/.node-red
6 Jul 19:24:25 - [warn] Projects disabled : editorTheme.projects.enabled=false
6 Jul 19:24:25 - [info] Flows file : /home/pi/.node-red/flows.json
6 Jul 19:24:25 - [info] Creating new flow file
6 Jul 19:24:25 - [warn] Using unencrypted credentials
6 Jul 19:24:26 - [info] Server now running at http://127.0.0.1:1880/
6 Jul 19:24:26 - [info] Starting flows
6 Jul 19:24:26 - [info] Started flows
